Credit Suisse is signing a deal to restack into 1.2 million square feet on the lower portions of 11 Madison Avenue, paving the way for Sony to lease roughly 520,000 square feet on the upper floors.

CS notified its employees Monday that a deal was completed to remain in the building. The Post has learned the financial giant signed a 20-year lease for the Art Deco granite and hourglass-shaped building opposite Madison Square Park where it already occupies 85 percent of the 2.3 million square feet.

The asking rent was in the $70s a square foot, sources said, which will total roughly $1.8 billion over that time. The new deal will include expansion options in the next few years as well as a renewal term.

Speaking from Belgrade, Serbia, where they were meeting with Prime Minister Ivica Dačić, Sapir Organization President Alex Sapir and CEO Rotem Rosen confirmed the transactions, which are still considered pending, but declined to discuss the financial terms. “This could be the biggest deal this year,” said Sapir.

The tenant was represented by Lewis Miller of CBRE, while CBRE colleague Howard Fiddle represented the Sapir ownership. Barry Gosin, CEO of Newmark Grubb Knight Frank, and Anthony Orso, CEO of CCRE, a “trusted advisor” of Sapir company founder and Chairman Tamir Sapir, advised the owners on the financial structure. The companies declined to comment.

CS is expected to temporarily shuttle employees through its space at the nearby One Madison Ave. while the lower floor offices are reconstructed.

The Sony transaction for the 19th to 28th floors is expected to be signed next week. Mary Ann Tighe, CEO of CBRE, is representing the tenant in its move from 550 Madison Ave., which was sold but provided time for it to identify a new location and have it fitted out.

WPP is another large tenant at 11 Madison, while Fidelity’s Investment Center opened last year in its retail space. The highly acclaimed Eleven Madison Park restaurant is in the retail space on the East 25th Street side of the building.